Automatic fidelity control



Sept. l22, 1936. J. s, sTARRETT AUTOMATIC FIDELITY CONTROL vFiled Nov. lO, 1933 IIIII IIL SEEE S n S SICE@ mmm w .N

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lAnother object of the invention is to provide Patented Sept. 22, 1936 AUTOMATIC FIDELITY CONTROL vJohn S. Starrett, Livingstom'N. J., assignor to Radio Corporation of America, a corporation of Delaware kApplication November 10, 1933, Serial No. 697,388

,e clin-uns.'v (o1. 25o- 20)v My present invention relates to radio receivers, and more particularly to transmission control circuits for radio receiver signals.

I-Iiss and high frequency noise reproduction which takes place along with the reproduction of high modulation frequencies constitute one of the chief diiiiculties experienced in high quality radio receivers. 'Ihese difficulties are usually encountered when receiving from distant stations, or Ythose providing a low field strength at the receiver. To eliminate, or minimize, these effects as has `commonly been the practice to supply a manual tone control, such as an 'electrical network, on the receiver, usually on the input or output of the -output tube. Such a tone control mechanism is varied to increase or decrease the attenuation of the lhigher audio frequencies. The usual form of a tone control networkin the prior art is a capacity of 0.01 to 0.05 microfarad connected in series with a variable 'resistor of 50,000 or 100,000V ohms, and this series circuit is usually bridged across the primary of the audio output transformer'. 1

l It is one of the main objects of the present invention to secure an automatic control of the aforementioned high audio'frequency attenuation function, a receiver embodying the present invention utilizing a photo-sensitive instrumentality in place of the variable resistorfusually employed in the tone control path.

Another important object of the present invention is to provide a means for automatically controlling the fidelity of reproduction of a radio receiver wherein a selenium cell -is utilized as a variable resistor in a tone control network disposed in the audio frequency amplifier network, the resistance of the cell being regulated by a .lightsource whose intensity varies with the automatically controlled bias on Vthe lhigh frequency network preceding the audio frequency in `a superheterodyne receiver a control arrangement which simultaneously regulates thei'gain of the radio frequency and intermediate frequency amplifiers of the receiver and the fidelity of reproduction of the'receiver, the fidelity control being accomplished by a photo-sensitive device functioningas a variable impedance in the audio frequency amplified network.

"trol andV fidelity control arrangements,V and especially to provideL a receiver of this type which` is not only reliable in operation, but economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, will best be understood by reference to the following description'taken in connection with the drawing, in which I have indicated diagrammatically a circuit organiza- 10 vtion whereby my invention may be carried into eiiect.

' Referring now to the accompanying drawing, which diagrammatically shows a conventional and well-known type of superheterodyne receiver, it is pointed out that the essential elements of the receiver include the usual grounded antenna circuit I, 'a tunable radio frequency amplifier 2, a combined rst detector-local oscillator 3, an intermediate frequency amplifier 4, :i210 a second detector 5, and an audio frequency amplifier 6. The first detector network 3, while described as employing a single tube for local oscillator and first detector functions, can, of course, be replaced by a network employing two 25 independent tubes for these two functions.

In any event,the tuning means of the amplifier 2 `and the network 3 are arranged for unicontrol adjustment, and this is designated by the dotted line l. The intermediate frequency out- 30 put of the network 3 is impressed upon the intermediateffrequency amplifier 4 through the tuned network 8, and the resonant network 9, tuned to the intermediate frequency, couples the amplifier'il to the second detector 5. The audio transformer I0 couples thesecond detector 5 to the audio amplifier 6, while the audio output transformer II couples the amplifier 6 to any wellfknown type of reproducer, such as a loud speaker.

Of course, one or more stages of audio frequency amplification may be disposed between the seconddetector 5 and the output tube 6, and

the tube 6 need not be of the pentode type. The

gain of the amplifiers 2 and 4 is regulated auto- 45 matically in an inverse sense with variations in carrier amplitude of received signals by means of fan automatic volume control tube I2, whose anode circuit is connected to the grid circuit of tube Zithrough a direct current connection I3. The direct current connection I4 connects the grid circuit of tube 4 to the anode of tube I2, and alsoto the grid of an automatic fidelity control tube I5.V The tube I2 has been designated as l55 AVC, and the tube I5 has been designated as (IA-B1G.

Signal energy is impressed upon the grid of tube I2 through a connection I6, which includes a radio frequency condenser I1. The cathode of tube I5 is connected to the cathodes of tubes 2 and 4 through a connection I8. The voltage supply source for the tubes I2, I5 and B is only shown because the voltage sources for the remaining tubes are conventional. The last named voltage supply source is not shown, but the bleeder connected across it is represented by the resistors I9, 20, 2l, 22, and 23. l I

The junction of resistors 20 and 2I is grounded, and the resistor 24 is connected between its ground point and the direct current connection I4. The cathode of tube i2 is connected between the junction of resistors I9 and 2,0, while' the cathode of tube I5 is connected between the junction of resistors 2l and 22. The anode of tubeul is connected to the junction ofV resistors 22 and 23 through a filament 25, the positive terminal side of resistor 23 being connected to the anode of the audio output tube 6 through the primary of the output transformer II. The grid and cathode of tube I2 are connected through resistor I9 and the resistor I9', and the junction of resistors I9 and I9 is connected to the negative side of the voltage supply source.

The tone control, or high audio frequency attentuation path, is connected acrossthe primary of transformer II, and this path includes a variable impedance Si) in series with a condenser 3l, Variable impedance 30 is a photo-sensitive element, such as a selenium cell, and is disposed adjacent the filament 25. It is believed only necessary to represent the selenium cell in conventional form, as its specific construction is well known tol those skilled in the art.

The selenium cell V3l) and the filament 25 are preferably sealed into a single bulb 32, and

`mounted in a standard four pin base, not shown.

The exterior, or interior, surface of the bulb 32 is coated with an opaque film 33 to' exclude light from outside sources. By way of illustration, it is pointed out that the condenser I'I may have a capacity of 0.001 microfarad, the resistor 24 may have a resistance of 0.5 megohm, the condenserV `3| mair have a capacity of 0,01 to 0.05 microfarad, and the variable resistor may have a resistance of 50,000 or 100,000 ohms.

The operation of the arrangement shown` will now be described, it being first pointed out that the chief Vobject is to secure automatic control of the attenuation path 30, 3l by controlling the resistance of element 30 from the light source 25, the intensity of the light admitted by filament 25 varyingrwitli the automatically controlled bias on the amplifiers 2 and 4. The AVC tube grid isfed with signal energy from the input of the second detector 5. Normally,that is with no signal input to the detector 5, the AVC tube I2 is biased to cut off. As the signal input through the condenser I'I increases, the space current Vflow through tube I2 increases, and, therefore, the plate current flow through the resistor 24 increases. The increase in plate current flow through resistorY 24 imposes a negative bias upon the grids of amplifiers 2 and 4. Thus, for a strong local signal, the direct current voltage across the resistor 24 will be from-35 to 40 volts, and with a Weak signal will be substantially Zero.

The voltage across resistor 2,4 also controls the plate current iiow through the fidelity control tube I5. Therefore, it will be seen that as the signal input increases the negative bias on tube I 5 increases, and the fiow of currentV through the plate circuit of tube I5 decreases. Accordingly, the heating of the filament 25 will vary with the variation in plate current iiow through tube I5. That is to say, the plate current of tube I5 controls the brilliancy of illumination of the lamp filament 25, the light from which falls upon the selenium cell 3). With a strong signal the lamp filament will be dark and the resistance of the selenium cell 3e high, and this means that the attenuation of the higher audio frequencies will be a minimum. For a weak signal, on the other hand, the illumination of the selenium cell becomes greater andthe cell resistance is lowered, with the result that the attenuation of the higher audio frequencies will be maximum.

It will, therefore, be seen that in the no-signal condition, the tube I5 has its maximum plate current fiow and the filament 25 emits maximum light. YThis is desirable, since, with no signals or weak signals coming in, the amplifiers 2 and 4 are operating with maximum efficiency and amplify background noise impulses to an annoying extent. In this condition the resistance of cell is at a minimum andthe attenuation of the higher audio frequencies is'at a maximum, which means that the hiss and high frequency noise reproduction is substantially eliminated, or

minimized. Y Y v While I haveY indicated and described one system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particu,- lar organization shown and described, but that many modifications may be made, without de.- parting from the scope of my invention, as set forth in the appended claims,

What I claim is:-

l. A background noise suppressor arrangement for a radio receiver which comprises atleast two successive high frequency amplifiers, aV detector and an audio frequency amplifier, said suppressor arrangement including an amplifier gain Control tube having a direct current connection between its anode circuit andthe grid circuits 0f said ampliers, a signal path between Said. detector and the grid circuit of the gain control` tube, a

Ynoise suppressor tube having its input electrodes connected to the anode circuit ,of said gain control -tube, a light source in the anode circuit of the suppressor tube, and an Yaudio frequency attenuation pathin the output circuit of the audio amplifier, said attenuation path including a photo-sensitive Yelement whose impedance depends upon the light emitted from said light source. Y

2. In an arrangement, as defined in claim 1, said photo-sensitive device consisting of a selenium cell, said suppressor tubeV havingvoltage constants such that the illumination fromsaid. light source is a maximum when substantially no: signals are impressed on the grdof Said gain control tube, whereby the impedance of said selenium cell is a minimum and the higher audio frequencies corresponding to undesired background noise impulses are byepassed out of the audio amplifier Voutput circuit. v

. 3. In combination with a signal transmission tube a demodulator, and av demodulated signal utilization network, means responsive to variations in received signal intensity forV regulating the gain of said tube in a sense to maintain the signal input tc the demoduiator substantially constant an: audio attenuation. path operatively ..15

associated with the utilization network, the path including a photo-sensitive element and means operative upon the gain of said tube becoming substantially a maximum, for varying the impedance of said element in a decreasing sense whereby the higher modulation frequencies are eiiiciently transmitted through Said path.

4. In combination with a radio receiver provided with a high frequency amplifier and a rectifier, an automatic volume control tube having a radio frequency signal connection between its grid and the rectifier, an automatic fidelity control tube having its input electrodes directly connected across an impedance in the plate circuit of the volume control tube, means in circuit with the input electrodes of the volume controltube for biasing the volume control tube substantially to cut-off when substantially no signals are impresed upon said rectier, an audio frequency network coupled to said rectifier, an audio frequency attenuation path connected to said network and including an element which has a variable impedance, and means in the plate circuit of said fidelity control tube for controlling the impedance of said element in said attentuation path, said means in the plate circuit of the fidelity control tube comprising. a source of illumination, and said variable impedance element comprising a photo-sensitive element.

5. In combination with a radio receiver provided with a high frequency amplifier and a rectifier, an automatic volume control tube having a radio frequency signal connection betwen its grid and the rectifier, an automatic fidelity control tube having its input electrodes directly connected across an impedance in the plate circuit of the volume control tube, means in circuit with the input electrodes of the volume control tube for biasing the volume control tube substantially to cut-off when substantially no signals are impressed upon said rectiiier, an audio frequency network coupled tosaid rectifier, an audio frequency attenuation path connected to said network and including an element which has a variable impedance, and means in the plate circuit of said fidelity control tube for controlling the impedance of said element in said attenuation path, said variable impedance element consisting of a photo-sensitive device, such as a selenium cell, and said means in the plate circuit of the fidelity control tube consisting of a light source, Whose illumination varies with the flow of the fidelity control tube plate current.

6. In combination with a signal transmisison tube, a demodulator, and a demodulated signal utilization network, means responsive to variations in received signal intensity for regulating 

